Microbial C and N in litter and soil of a southern beech (Nothofagus) forest: Comparison of measurement procedures

The suitability of fumigation-incubation and fumigation-extraction procedures for measuring microbial C and N in strongly acid litter and different depths of soil of a southern beech (Nothofagus spp) forest was investigated. The substrate-induced respiration (SIR) method was also briefly examined. In the fumigation-incubation method, a large inoculum (20% w/w) of unfumigated sample and controls of either fumigated L and FH materials or unfumigated mineral soil incubated for 10–20 days gave maximum CO2-C flush values; controls of unfumigated material incubated for 0–10 days were unsuitable. The kc-factors selected for conversion of CO2-C flush to microbial C depended on sample pH and ranged from 0.28 to 0.44. Use of no control, and a common kc-factor of 0.45, appeared to over-estimate microbial C in L and FH samples, but gave reasonable estimates in the samples of mineral soil. Inoculum size had no detectable influence on mineral-N flush, determined with an unfumigated control. In the fumigation-extraction procedure, increasing fumigation time from 1 to 5 days significantly increased all extractable-N, but not extractable-C, values. Means of experimentally determined kEC-factors, for converting extractable-C flush to microbial C, varied from 0.28 to 0.31 (SD, 0.05–0.09), depending on which 10–20 day control was used in calibration with the fumigation-incubation method. The kEN-factors when calculated in a similar way appeared unrealistically high. The SIR method appeared to under-estimate microbial biomass, when a conventional formula was used for converting CO2 produced to microbial C. Further calibration of the SIR procedure, however, seems warranted. The fumigation-extraction procedure is generally recommended for measuring microbial C and N in these strongly acid samples, using a kEC-factor of c 0.30 and the provisional kEN-factor of 0.45, respectively.